1. Field of the Invention
The present invention relates to a new process for the removal of water from aqueous acrylic acid solutions.
2. Discussion of the Background
Acrylic acid can be prepared on a large industrial scale in various ways, the processes depending on the starting substance. According to U.S. Pat. No. 2,922,815, acrylic acid can be prepared from acetylene, carbon monoxide and water. In this process, the water remains as an excess in the product stream. Water is also used in the hydrolysis of acrylonitrile to acrylic acid according to U.S. Pat. No. 2,734,915. Excess water is consequently also obtained here, in addition to the product. Another possibility is, in accordance with U.S. Pat. No. 4,537,874, atmospheric oxidation of propylene to acrolein and subsequent further oxidation to acrylic acid. The reaction temperature is lowered drastically by supplying water (quenching), in order to end the reaction. Water is likewise obtained as a byproduct in this process. In the various processes, the product must subsequently be separated not only from the gaseous components which have not reacted but also from the water supplied or formed.
Various process variants have been proposed which realize removal of water from acrylic acid. If the water content is quite high, direct distillation of the water may prove to be uneconomical. Various processes are known for effecting the removal of water by an extraction step, extraction agents being, for example, ethyl acetate (GB 995 472), diisopropyl ether (GB 1 081 095), alkyl acrylates, ketones, .beta.-alkyloxypropionate, derivatives of cyclohexanone (U.S. Pat. No. 3,657,332) or pyrrolidone. Acrylic acid is absorbed by the extraction agent and is recovered therefrom by distillation or by means of extractive distillation, crystallization or azeotropic distillation. When the water content is less than 60% in the starting mixture, the extraction step can be bypassed and water can be removed from the solution directly by azeotropic distillation. However, because of the exposure to heat, there is then also the risk of obstruction (clogging or blocking) due to polymerization, even if the acrylic acid is stabilized. Distillation at a low temperature under normal pressure would furthermore lead to high volumes of vapor and subsequently to large columns.
A preliminary process stage which reduces the water content in the solution significantly, for example to less than 40%, under mild conditions would be appropriate, so that the removal of water by distillation is limited to only a low water content. Pervaporation is a method in which a semipermeable membrane is introduced as a barrier for the organic substance in the transition from the liquid to the vapor phase. Since the membrane preferentially allows the passage of water, the separating action is promoted significantly. Numerous applications of pervaporation have been disclosed in recent years, for example for removal of water from organic solvents, alcohols and amines, a membrane which is essentially a separating layer of polyvinyl alcohol (PVA) allows selective removal of the water as a vapor phase.
According to Q. T. Nguyen (1991), Proc. Int. Conf,. Pervaporation Processes Chem. Ind. 5th, 67-78, customary PVA membranes are, however, unsuitable for pervaporation of aqueous acetic acid above 80.degree. C., since the membrane in this case dissolves. The same also applies in particular to the even more aggressive aqueous mixture of acetic acid and chloroacetic acid. These strong organic acids can be concentrated only with the aid of special membranes. A special membrane of PVA, polyacrylate and polysulfone is accordingly used in U.S. Pat. No. 4,971,699 for pervaporation of aqueous formic or acetic acid. According to M. Yoshikawa (1993), J. Membrane Sci., 82, 157-62, aqueous acetic acid can also be pervaporated with the aid of a membrane of acrylic acid/acrylonitrile copolymers.
Pervaporation of aqueous acetic acid or an even stronger aqueous acid with the aid of PVA membranes is accordingly not recommended.